Suppression of schlieren phenomena



May 5, 1970 u. LA ROCHE SUPPRESSION ,QF SCHLIEREN PHENOMENA 2 Sheets-Sheet 1 SUCTlON KIA/ab I May 5, 1970 u. LA ROCHE SUPPRESSION OF SCHLIEREN PHENOMENA 2 Sheets-Sheet 2 Filed Aug. 15, 1967 United States Patent SUPPRESSIGN 0F SCHLIEREN PHENOMENA Ulrich La Roche, Zurich, Switzerland, assignor t0 Contraves AG, Zurich, Switzerland Filed Aug. 15, 1967, Ser. No. 661,173 Claims priority, application Switzerland, June 23, 1967, 11,7 58/ 66 Int. Cl. G02!) U.S. Cl. 350-63 8 Claims ABSTRACT OF THE DISCLOSURE Wall means defines an enclosed space which has a predetermined inner temperature. An observation window is mounted in the wall means exposed to this inner temperature and has a surface which is located outside the space and is exposed to a body of fluid having a different temperature. A nozzle structure is arranged adjacent the exposed surface and defines a fluid channel extending normal to the exposed surface and having an inner end adjacent to the exposed surface, and an annular gap communicating with the fluid channel so that, when suction is applied to the annular gap, fluid will be drawn through the channel in the direction against the exposed surface of the observation window and will thereupon be caused to flow across the observation window in radial direction and toward the marginal zones of the exposed surface.

BACKGROUND OF THE INVENTION It is well known that density variations occur at the nterface between a body of fluid and an exposed surface if a temperature differential exists between the two. This leads to the formation of schlieren, a term which has become accepted for describing a phenomenon in which the density variations in the fluid adjacent the surface appears to be developing oscillating streaks.

This phenomenon can be very undesirable under certain circumstances, for instance if it is necessary to conduct observations through that zone of the fluid in which the phenomenon has developed. Particularly in cases where relatively small objects are to be observed, tracked or located, for instance observation and location of flying bodies at great distances by means of optical observation and/or tracking devices, the results obtained can be significantly affected in an adverse manner if such observation must be made through a schlieren-containing zone or region of fluid. It is of course known that there are differences in the size of schlieren, some of which are relatively small while others are much larger. It is particularly the relatively large schlieren which have been found objectionable under the circumstances just discussed because they decrease the contrast in the observation of relatively small objects or of objects which move rapidly with respect to their size.

The present invention overcomes the aforementioned problems. More particularly, the present invention inhibits the development of schlieren, and particularly of the relatively large schlieren, which, as mentioned before, have been found objectionable in optical observations.

In accordance with one feature of my invention, I provide a method of inhibiting in a body of fluid which contacts the exposed surface of a solid body, the formation of the schlieren resulting from the existence of a temperture differential between the surface and the fluid. My novel method comprises inducing the flow of a stream of the fluid in the direction against the exposed surface, and deflecting the stream of fluid adjacent the exposed surface so that it will flow in a plurality of directions along the surface and towards the marginal zones of the same.

My invention resides, however, not only in the aforedescribed method, but also in an arrangement for carrying out this method, and in this arrangement I provide wall means which defines an enclosed space having a predetermined inner temperature, and an observation window mounted in the wall means exposed to the predetermined temperature. The observation window has a surface located outside the space and being exposed to a body of fluid which has a different temperature than the temperature prevailing within the enclosed space. In accordance with my invention, I provide inhibiting means which is arranged outwardly adjacent to the surface and which is constructed and arranged so as to affect the flowing of a stream of the fluid initially in the direction against the surface and subsequently in a plurality of directions along the surface toward the marginal zones thereof. The stream of fluid will advantageously be caused to fiow in the direction normal to the exposed surface of the observation window, and will then be withdrawn in radial direction along the exposed surface toward the marginal zones thereof, advantageously by being subjected to suction. In so doing I force the boundary layer of fluid which is located adjacent the exposed surface to flow therealong at relatively high speed. Whereas free and unaffected flow of the fluid along the exposed surfacethat is without the benefit of the arrangement according to my inventionresults in the unhindered formation of schlieren in the boundary layer, application of my novel invention inhibits the schlieren formation and reduces the median size of the schlieren, that is of the turbulent inclusions which develop in the boundary layer as a result of the existing temperature differential between the fluid and the exposed surface. Thus, the development of the particularly undesirable large schlieren is prevented. Of course, it is necessary to assure that the rate of flow of boundary layer fluid, which is occasioned in accordance withthe present invention, is not so great as to result in the formation of turbulence.

If this condition is met in utilizing my invention one obtains on viewing of an object through an observation window protected according to my invention a modulatin transfer characteristic which, because of the existence of only small schlieren in the boundary layer, affords a contrast for high frequencies, i.e., relatively small objects, which to all intents and purposes is no worse than if no turbulence existed at all in the boundary layer fluid.

A particularly important area of application for my novel invention is optical observation and/or measuring devices, particularly those which are used for observing and/or ranging of relatively small objects at relatively great distance. In such devices the most sensitive optical and/or mechanical elements are arranged in a housing which frequently is maintained at a precisely controlled internal temperature and which is provided with an observation window extending transversely of the optical observation axis to which it is coaxial. It is this observation window which is to be protected against the formation of the schlieren phenomenon of the type discussed above.

The novel features which are considered as characteristic for the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 illustrates, in fragmentary section, one embodi ment of the invention in its most simple form;

FIG. 2 is a view similar to FIG. 1, but illustrating a modified embodiment;

FIG. 3 illustrates yet a further embodiment of the invention, also in a view substantially similar to FIG. 1; and

FIG. 4 illustrates yet an additional embodiment of the invention in partly-sectioned, partly-diagrammatic form.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Discussing now the drawing in detail, and firstly FIG. 1 thereof, it will be seen that reference numeral indicates an optical observation window which is mounted in a wall means 11. The wall means 11 should be understood to define or constitute a part of, a housing enclosing an internal space. The optical axis of the observation window 10 is indicated by the line AA and arranged outwardly adjacent the outer exposed surface of the observation window 10, there is arranged inhibiting means for inhibiting the formation of schlieren, and particularly of large-sized schlieren.

This inhibiting means comprises, as FIG. 1 clearly shows, an annular body 12 which defines a channel 12a. FIG. 1 shows that the channel 12a is coaxial with the optical axis A-A of the observation window 10. An inwardly directed surface 12b of the body 12, that is an annular surface which faces the wall means 11, defines with the same an annular gap 13 which is open in the direction towards the optical axis AA, so that it communicates with the inner end of the channel 12a. The gap 13 further communicates with a conduit 14 which is connected to a diagrammatically illustrated suction device. The latter need not be discussed in detail because suitable suction devices are well known to those skilled in the art and because the invention does not reside in details of such a device.

It will be understood that a body of fluid, usually air, is outwardly adjacent the wall means 11 and the outer exposed surface of the observation window 10, so that it surrounds the member 12. With the suction device in operation, a stream of this air will be aspirated through the channel 12a in the direction of the arrows and towards the exposed surface of the observation window 10. In the region or vicinity of this exposed surface the flow of air changes direction because of the suction exerted on it and flows across the exposed surface of the observation window 10 in radially outward direction towards and beyond the marginal Zones of the exposed surface. Of course, new air is constantly being aspirated through the channel 12a in the direction of the arrows and it must be remembered that the suction exerted must be such that this new air will be aspirated with the minimum of possible turbulence as already indicated earlier.

In the embodiment illustrated in FIG. 2, the observation window is again assumed to be an optical observation window and is identified with reference numeral 20. The wall defining the inner enclosed space, and on which the observation window is mounted, is identified with reference numeral 20a. Member 22 corresponds to the member 12 of FIG. 1 and defines with the marginal zones of the exposed surface of the observation window 20 an annular gap 22b to the gap 13 shown in FIG. 1. Unlike the embodiment of FIG. 1, however, the gap 22b has an annular filter 23 arranged therein. This may be in the form of a strip of filter material which is, of course, airpermeable. The purpose of the filter 23 is to assure even and turbulence-free withdrawal of the air at all points of the circumference of the gap 22b.

The embodiment of FIG. '2 differs from that of FIG. 1 in yet another respect in that member 22 is provided adjacent the inner end of the passage 22a with an annular slot 25 which surrounds that portion of the member 22 which defines the channel 22a. The slot 25 also communicates with the aspiration conduit 24, just as does the gap 22b, and the purpose of providing the slot 25 is to withdraw the boundary layer of air from the vicinity of the member 22 and of the wall means 20a so as to prevent it from being aspirated into the inlet end of the channel 22a. The reason for this, is of course, that this boundary layer air has already schlieren formed therein so that its aspiration into the channel 22a is not desirable.

Corning now to the embodiment of FIG. 3, it is to be understood that reference numeral 3 indicates a telescope tube on which there is mounted, for instance by being slid over the free end of the tube 3 and being frictionally retained thereon, an arrangement which comprises an optical observation Window 30 outwardly adjacent to whose outer exposed surface there is arranged a member 32 corresponding to the members 12 in FIG. 1 and 22 in FIG. 2. The member 32 defines an aspirating channel 32a corresponding to the channels 12a and 22a of FIGS. 1 and 2, respectively. It further defines an annular gap 32b corresponding to the gap 22b of FIG. 2, and the gap 32b has disposed therein a filter element 33 which corresponds to the filter element 23 in FIG. 2 and which is, of course, air-permeable. The annular gap 32b communicates with a suction or aspiration conduit 34 which in turn is connected with a suitable suction device, such as shown in FIG. 1. With this embodiment, in which the free end of the telescope tube 3 is closed off by the observation window 30 and the mounting therefor, the interior of the telescope tube 3 can be maintained at a desired and preferably constant temperature, for instance by filling it with an extremely light gas, such as helium, and circulating this gas in the tube 3. The boundary layer which develops at the outer exposed surface of the window 30 and tends to form schlieren as a result of the existing temperature differential, is withdrawn through the gap 32b.

The embodiment of FIG. 3 also comprises means for preventing the aspiration of schlieren-containing boundary layer to the free end or inlet end of the channel 32a, the reason being the same as set forth in FIG. 2. Unlike the embodiment of FIG. 2, however, the embodiment of FIG. 3 achieves this prevention of schlieren-containing boundary layer air not via a slot corresponding to the slot 25, but via a radially outwardly projecting flange 320 which extends relatively far away from the member 32 and thus guides the schlieren-containing boundary layer away from the inlet end of the channel 32a.

Coming, finally, to the embodiment of FIG. 4, it is pointed out that reference numeral 50 is intended to identify a photo theodolite which rests on a platform 5 turnable about a vertical axis V. The instrument 50 is provided with an objective-tubus 51 which can be turned about a horizontal axis H. A dome-shaped housing 52 surrounds the instrument 50 and is secured to the platform 5 so as to be turnable with the same and, therefore, with the instrument 50.

The housing 52 is provided with a meridional slot which is covered by a curved closure member 53. The member 53 is operatively associated with the instrument 50 so as to slide along the meridional slot of the housing 52 in correspondence with elevational movements of the tubus 51 about the axis H. In other words, whenever the tubus 51 swivels in the direction of the double-head arrow, the closure member 53 will slide out of the way. As FIG. 4 shows, the closure member 53 carries an optical observation window 40 whose optical axis AA is always in alignment with the optical axis of the tubus 51 by virtue of the fact that the operative connection between the drive means for the tubus 51 and the drive means for the closure member 53 effects movements of the two in such a manner as to maintain this axial alignment. All of this is course well known and need not be further described. It has been outlined here only for the sake of completeness of description. A roller-type closure or curtain 54 has a free end secured to the closure member 53 so that it is rolled up when the closure member 53 moves towards the left and downwardly in the picture, and is upwardly withdrawn if the closure member moves in the opposite direction. Thus, whatever portion of the meridional slot of the housing 52 is exposed by the closure member 53 is immediately closed by the curtain 54.

Unlike the preceding embodiments of the invention, the observation window 40 is provided with my novel arrangement adjacent both its outer and inner exposed surfaces. Thus, a body 42 corresponding to the body 32 of FIG. 3 is arranged adjacent the outer exposed surface of the observation window 40 and defines therewith an annular gap such as that illustrated in FIG. 3, which has disposed therein an air-permeable annular filter 43 corresponding to the filter 33 of FIG. 3. The annular gap communicates with an aspiration conduit 44 which in turn is connected to a suction device as illustrated in FIG. 1.

Arranged adjacent the inner exposed surface of the observation window 40, that is the surface which faces the interior of the housing 52, is a second nozzle body, identified with reference numeral 42a, which defines a further annular gap which also has disclosed therein an annular filter 43a. This gap communicates with aspiration conduit 44a which is connected to the same suction means as that to which the conduit 44 is connected, or to a separate suction means. The purpose of this arrangement is to assure that the boundary layer air is withdrawn from both the inner and outer exposed surfaces of the observation window 40, to eliminate the formation of schlieren, thereby making it possible to maintain the entire interior space within the housing 52 at constant temperature without danger of the development of schlieren.

It is to be expressly understood that, while the embodiments here illustrated all utilize my novel invention in conjunction with an optical observation device or arrangement, I do not Wish my invention and my inventive concept to be so limited. It is clear that my invention has a much wider range of applicability than merely in preventing the formation of schlieren adjacent the observation windows of optical observation instruments of arrangements.

It will be understood that each of the elements described above, or two or more together, may also find a useful application in other types of arrangements differing from the types described above.

While the invention has been illustrated and described as embodied in arrangements for suppressing schlieren phenomena, it is not intended to be limited to the details shown, since various modifications and structural changes may be made without departing in any way from the spirit of the present invention.

Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can by applying current knowledge readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention and, therefore, such adaptations should and are intended to be comprehended within the meaning and range of equivalence of the following claims.

What is claimed as new and desired to be protected by Letters Patent is:

1. A schlieren inhibiting arrangement, particularly in conjunction with optical apparatus, comprising, in combination, wall means defining an enclosed space having a predetermined inner temperature therein; an observation window mounted in said wall means with the inside surface thereof exposed to said predetermined temperature and having a surface located outside said space and exposed to a body of fluid having a different temperature than said predetermined temperature; inhibiting means outwardly adjacent to said outside surface for effecting the flow of a stream of said fluid from a region remote from said surface initially in direction against said surface and subsequently in a plurality of directions along said surface toward the marginal zones thereof, said inhibiting means comprising an annular suction channel extending along the marginal zones of said surface concentric therewith and having radially inwardly facing suction aperture means, and nozzle means for directing from a region remote from said outside surface a substantially turbulence-free stream of fluid in direction axially of said suction channel against the center of said outside surface; and suction means connected to said annular suction channel for withdrawing through said radially inwardly facing aperture means the fluid directed against the center of said outside surface in radial direction along said surface and into and out through said annular suction channel.

2. Arrangement as defined in claim 1, said window being an optical observation window and having an optical axis, and wherein said suction channel is coaxial with said optical axis.

3. Arrangement as defined in claim 1, and further comprising fluid-permeable filter means covering said annular gap whereby to minimize the occurrence of turbulence in the fluid aspirated into said annular gap.

4. Arrangement as defined in claim 1, and further comprising preventing means for preventingt he aspiration into said nozzle means of boundary-layer fluid from the vicinity of said wall means and in which schlieren have developed.

5. Arrangement as defined in claim 4, said nozzle means having an outlet end adjacent said exposed surface and communicating with said annular gap, and an inlet end spaced from said outlet end and from said exposed surface; and wherein said preventing means comprises a flange extending radially of said nozzle means in the region of said inlet end.

6. Arrangement as defined in claim 4, said nozzle means having an outlet end adjacent said exposed surface and communicating with said annulargap, and an inlet end spaced from said outlet end and said exposed surface; and wherein said preventing means comprises slot means provided in said nozzle means and communicating with said annular gap for aspirating therethrough and into said annular gap schlieren-containing boundary-layer fluid so as to prevent aspiration of such fluid into said inlet end.

7. Arrangement as defined in claim 6, wherein said slot means comprises an annular slot.

8. Arrangement as defined in claim 1, wherein said window has an additional exposed surface facing the interior of said space; and additional inhibiting means similar to the first-mentioned inhibiting means inwardly adjacent to said additional exposed surface.

References Cited UNITED STATES PATENTS 3,103,712 9/1963 Allinikou et al. 3503l9 3,118,231 1/1964 Rathsmill 35063 3,310,356 3/1967 Borberg 350-63 FOREIGN PATENTS 119,777 3/ 1949 Australia.

PAUL R. GILLIAM, Primary Examiner 

